Over the past few decades, the need of powering e.g. wireless sensors has initiated the research activity on energy harvesting from the environment. The wireless communication capabilities cannot be fully utilized if energy is supplied by wires, or if there is a battery with limited operation time, bulky size, and large weight.
The energy harvesting mechanism depends on the environment and the application. Various energy sources have been proposed: light, heat (thermal gradient), strain, vibrations, electromagnetic field, kinetics, air flow, pressure variations, and radioactive materials, as well as also vibration.
One of most studied application for energy harvesting is e.g. car tyre pressure monitoring systems TPMS. There are already battery operated devices on the market, but harvesters would increase the operation time of these systems. The power level needed for TPMS is currently in the range of 100 μW. This is at least ten times more than reported by the typical state-of-the-art devices published. However, the power levels required by the applications are constantly decreasing while the harvested power levels are increasing. It seems likely that one day these levels will meet.
There are known some prior art energy harvesting devices, such as for example US 2007/0125176 A1 discloses an energy harvesting device having a micro-electromechanical structure fabricated as a plurality of members respectively resonant at different frequencies so that the structure can respond to a number of different vibration frequencies. Piezoelectric material converts the vibrations into an electric voltage difference across at least a portion of the structure.
In addition WO 2007044443 discloses a solution of powering an electronic device in a tire monitoring system using a tire pressure based energy scavenger, where pressure changes caused by the rotation of the tire are converted into electrical energy with a transducer having a casing which surface is coated with a conductive layer (1112), and an ultra-thin (approximately 100 nm) dielectric film (1114) to ensure that the conductive diaphragm never actually makes electrical contact with the conductive layer on the rigid casing.
WO 2007044443 also teaches a variable capacitor transducer as a pressure-based energy scavenger, where a rigid casing (1110) is used to create an air-gap variable capacitor between elastomer diaphragm (1120) and rigid casing (1110), and where the variable capacitor is not created by the dimensional change of the diaphragm, but by the changing gap between the diaphragm and the rigid casing. The surface of the diaphragm needs to be conductive.
Furthermore WO 2007121092 A1 discloses a piezoelectric power generator capable of harvesting energy from environmental vibration. The generator includes a dielectric frame supporting a piezoelectric panel having an upper electrode and a piezoelectric layer formed over a lower electrode and a dielectric layer and an end mass formed on the piezoelectric layer. The end mass provides weight to cause the piezoelectric panel to move (vibrate) within the frame and causes the generation of electrical power. The generator is preferably formed by a MEMS process.
However, some disadvantages relate to the known prior art solutions. For example most of the energy harvesters function efficiently only for a certain resonance frequency. In addition devices designed to responding to a number of different vibration frequencies typically consist of a plurality of members respectively resonant at different frequencies. However, the devices with plurality of members become quite complex to manufacture. In addition capacitive based energy harvesters need a battery for a bias voltage in order to start operating properly. Also many of the known harvesters are vulnerable to vibration exceeding the limits of the intended use.